Field of the Invention
The present invention relates to a liquid developer for use in image forming apparatuses that utilize an electrophotographic system, for example, electrophotography, electrostatic recording, and electrostatic printing.
Description of the Related Art
An electrophotographic system is a method in which printed material is obtained by uniformly charging the surface of an image bearing member such as a photosensitive member (charging step), forming an electrostatic latent image by photoexposure of the surface of the image bearing member (photoexposure step), developing the thereby formed electrostatic latent image with a developer that contains colored particles (development step), transferring the developer image to a recording medium such as paper or plastic film (transfer step), and fixing the transferred developer image to the recording medium (fixing step).
The developers here are broadly classified into dry developers and liquid developers: colored particles constituted of a material that contains a binder resin and a colorant such as a pigment are used in a dry state in the former, while the colored particles are dispersed in an electrically insulating liquid in the latter.
The need for color output and high-speed printing from image forming apparatuses that use an electrophotographic system, e.g., copiers, facsimile machines, printers, and so forth, has been increasing in recent years. Within the realm of color printing, the demand for high-resolution, high-quality images has resulted in demand for developers that can accommodate high-speed printing while having the ability to form high-resolution, high-quality images.
Liquid developers are known to be developers that offer advantages with regard to color image reproducibility. With a liquid developer, the occurrence of aggregation by the colored particles in the liquid developer is suppressed, and due to this a microfine toner particle can be used. As a consequence, excellent properties with regard to the reproducibility of fine line images and the reproducibility of gradations are readily obtained with a liquid developer. Development is becoming quite active with regard to high-image-quality, high-speed digital printing apparatuses that exploit these excellent features by utilizing electrophotographic technologies that use liquid developers. In view of these circumstances, there is demand for the development of liquid developers that have even better properties.
Investigations have been carried out into various charge control agents for charging the toner particles in liquid developers and carrying out development by and transfer of the developer by electrophoresis.
For example, it is proposed in Japanese Patent No. 3,267,716 that low molecular weight compounds such as lecithin are effectively used as toner charge control agents that bring about negative charging by a toner.
In Japanese Patent Application Laid-open No. H7-319223, on the other hand, it is proposed that a polymer compound containing a quaternary ammonium salt functions as an effective negative-charging toner charge control agent.
In addition, Japanese Patent Application Laid-open No. H7-261467 indicates that an analogous polymer compound containing a quaternary ammonium salt, through its incorporation in the toner particle, functions as a charge adjuvant for causing the toner to undergo positive charging.